Light-Emitting Diodes or “LEDs” are increasingly being used for general lighting purposes. In one example, a group of so-called white LEDs is powered from an AC power source. The term “AC LED” is sometimes used to refer to such circuits. AC LED circuits that utilize opposing strings of parallel-connected LEDs are described in the following U.S. Pat. Nos. 5,495,147, 6,830,358, 7,045,965, 7,138,770, 7,264,381, and 7,344,275. AC LED circuits involving bridge-rectified LED strings are described in the following U.S. Pat. Nos. 5,463,280 and 7,276,858. U.S. Patent Application Publication US2007/0008721 shows a network of parallel-connected AC LEDs connected in multiple ways.
FIG. 1 (Prior Art) is a diagram of one AC LED lamp circuit that requires neither an AC-to-DC converter nor a rectifier. Although a DC voltage can be supplied, an AC voltage is typically supplied between input terminals 1 and 2. Terminals 1 and 2 may, for example, be coupled to receive 110V AC line power. If the voltage on terminal 1 is higher than the voltage on terminal 2, then current flows from terminal 1, through current-limiting resistor 3, to node 4, then through a first string of series-connected LEDs 5 to node 6, then through current-limiting resistor 7, and to terminal 2. If the voltage on terminal 1 is lower than the voltage on terminal 2, then current flows in the opposite direction from terminal 2, through current-limiting resistor 7 to node 6, through a second string of series-connected LEDs 8 to node 4, then through current-limiting resistor 3, and to terminal 1.
FIG. 2 (Prior Art) is a diagram of a second AC LED lamp circuit. This circuit employs a full-wave rectifier 9. A DC or AC signal is received on terminals 10 and 11. Terminals 10 and 11 may, for example, be coupled to receive 110V AC line power. On each half-cycle of an incoming AC signal both LED strings 12 and 13 emit light, but the extra cost of providing the full-wave rectifier 9 is involved. As in the case of the circuit of FIG. 1, current flow is limited by providing current-limiting resistors 14 and 15.
Effective use of the AC LED circuits of FIGS. 1 and 2 generally requires good control and matching of the line in AC voltage to the voltage drop across the LEDs to ensure stability, and/or requires adding current-limiting resistors as illustrated to limit the current variation as the line voltage or LED voltage drop changes. A disadvantage of the current-limiting resistor approach is power loss. This power loss results in lower efficiency of the LED lamp and higher heat generation.
In a 110V AC operational example of the circuit of FIG. 2, an effective 625 ohm resistance is disposed in series with the LED strings operating at 110V AC and forty milliamperes RMS. This results in one watt of resistive power loss for a four watt LED lamp. This amounts to a twenty-six percent loss of efficiency. In a 220V AC operational example, an effective 2.5 k ohm resistance is disposed in series with the LED strings operating at 220V AC and twenty milliamperes RMS. This results in one watt of resistive power loss for a four watt LED Lamp. This amounts to a twenty-six percent loss of efficiency. Because the RMS voltage drop across the current-limiting resistances is approximately twenty-six percent of the AC line RMS voltage, a ten percent increase in the AC line voltage results in approximately a forty percent increase (10 percent/26 percent) in LED current. This increase in LED current causes seventy percent more power loss across the resistances, causes increased heat dissipation, and causes a roughly fifty percent increase in power consumption. Likewise, a ten percent reduction in line voltage results in an observable thirty-six percent drop in LED current.
To avoid such resistive power loss, capacitor-coupled AC LEDs have been proposed. For example, U.S. Pat. Nos. 6,972,528 and 7,489,086 disclose using a capacitor in series with parallel-connected opposing LED pairs for high frequency decoupling. A disadvantage of this approach is the need to provide a high value capacitor. Another disadvantage is its reliance on a stable dV/dt and its general incompatibility with triac dimming. Such a circuit can be driven with a high frequency AC source controller, but doing so requires complicated circuitry for the high frequency driver.
U.S. Pat. No. 6,577,072 sets forth another approach that uses non-monolithic circuitry comprising a switch connected to a string of LEDs in parallel with storage capacitors. The switch is turned off when the line voltage drops below a certain level so that the LEDs are supplied by the capacitors. This is not a loss-less approach because the drained capacitors require recharging through the switch. Furthermore, light output of the LEDs is not regulated against changes in line voltage.